A cyclic pressure pulse generator for downhole application based on charges consisting of propellant layers burning sequentially with alternating rates was developed. Layers consist of loose-packed particulate mixtures of solid fuel, solid oxidizer and hydrocarbon functional additive.
There are several traditional approaches for formation treatment: acidizing and hydraulic fracturing; they are based on pumping of high volumes of treatment fluid to the well.
The disclosed device and method relate to the impulsive method of formation stimulation. The device induces creation of numerous cracks/fissures in the subterranean formation. This method can be considered as independent treatment or used in combination with traditional treatments, e.g., as a prerequisite stage to hydraulic fracturing.
Existing vibro-cracking models demonstrate that the impact of pressure pulses with a higher frequency and amplitude (better at the level of tens of MPa) produces massive spalling in the near-wellbore zone, and if the well has a fracture already, this creates new cracks spreading outward from existing fracture. It appears to be quite difficult to attain pressure pulses of sufficient magnitude and required frequency by conventional mechanical devices in practical application of this model.
On the other hand, as reported in [Pioneering new concepts in wireline conveyed stimulation and surveillance. Hi-Tech Natural Resources, Inc, 1991; Swift R. P., Kusubov A. S., Multiple Fracturing of Boreholes By Using Tailored-pulse Loading, SPE Journal, 1982, N 12, pp. 923-932] even without cyclic pulsing, multiple radially oriented fractures may be formed provided the fast rise of fracture-forming stress, in excess of 104 MPa/s.
Hence, development of pulse treatment for pay reservoir necessitates search for a design of the pressure pulse source that combines opportunities of a cycle of pressure pulses and flexibility of amplitude and time parameters, while keeping a higher power of total impact.
Burning of fuel oxidizer compounds, e.g. particulate mixtures based on ‘metal fuel-solid oxidizer-liquid additive’ type compositions might be considered a way of producing pressure pulses of required characteristics. This approach provides several positive outcomes:    (a) possibility to attain pulsing regime by controlling burning velocity, e.g. varying mixture composition, size of particles, and charge porosity (density):    (b) high energetics due to presence of metal particles hence providing charge compactness;    (c) possibility to adjust pressure pulse profile and place of impact by providing conditions for partly water reacting charge, namely providing rich mixture, that would react downstream the injection trajectory;    (d) little or no shattering or compaction of the formation.
Energetic materials in general are capable of a dual reacting regime:                supersonic regime: a combustion wave preceded by a strong shock wave brings about a detonation wave, propagating at a speed on the order of several km/s and limited by the total thermochemical energy content of the reacting material;        subsonic regime: a combustion wave brings about a deflagration wave, propagating at a velocity on the order of cm/s and limited by heat and mass transfer processes.        
The disclosed method describes the use of imperfect mode of charge combustion which is close to the subsonic mode, but still able to produce strong shock waves. The physical and chemical properties of the mixed charges dictate the convective mode of combustion.
Convective burning is a special sort of burning in porous energetic materials, sustained and propagated due to convective heat transfer from hot burning products. Burning products penetrate into pore spaces of the charge and provide conditions for heating and ignition of energetic material at pore surfaces [A. F. Belyaev and V. K. Bobolev, Transition from Deflagration to Detonation in Condensed Phases (National Technical Information Service, Springfield, Va., 1973); Sulimov A. A., Ermolaev B. S. , Chem. Phys. Reports, 1997, V.16(9), pp. 1573-1601; Sulimov A. A., Ermolaev B. S., et al. , Combustion, Explosion and Shock Waves, 1987, Vol. 23, N.6, pp. 669-675; E. P. Belikov, V. E. Khrapovskii, B. S. Ermolaev and A. A. Sulimov, Combustion, Explosion and Shock Waves, 1990, V.26, N.4, pp. 464-468].
The characteristic feature of convective burning is a wide range of combustion wave velocity: from several meters per second up to several hundred meters per second. The wave velocity depends on the following parameters:                properties of mixture components (energy density, temperature for particle ignition, particulate size, etc.);        properties of charges (geometry, composition, porosity, heterogeneity and layers in the charge assembly);        initial conditions (temperature and pressure).        
The possibility to control convective combustion and obtain reproducible parameters of pulses for a desired range of velocity and pressure had been checked in [E. P. Belikov, V. E. Khrapovskii, B. S. Ermolaev and A. A. Sulimov, Combustion, Explosion and Shock Waves, 1990, V.26, N.4, pp. 464-468; Sulimov A. A., Ermolaev B. S., Belyaev A. A, et al., Khimicheskaya Physika, 2001, V.20, N.1, p.84]. This demonstrated that the convective combustion is quite attractive as a tool for pressure pulse generation.
We should note that up to now the researches have been performed experiments mainly for gun powder systems without metal fuel additives (e.g., aluminum) or only for the single-pulse mode.
For the disclosed design of the cyclic pressure pulse generator, the preferred composition of combustion mixtures is a solid fuel and solid oxidizer, e.g., a mix of aluminum powder, ammonium nitrate or perchlorate with additive of kerosene or nitromethane. However, other combustion mixtures can be used: the metal powder can be substituted by coal powder, poly(methyl methacrylate) (PMMA) powder.
Experiments [Sulimov A. A., Ermolaev B. S., Belyaev A. A, et al., Khimicheskaya Physika, 2001, V.20, N.1, p.84] confirmed the practical possibility to achieve convective combustion of mixtures comprising ammonium perchlorate and aluminum powder. Experiments were carried out in a constant-volume bomb setup for tracking the initiation and development of convective combustion in this type of mixture.
The prior art in oil production industry teaches that the compositions of metallic fuel with the perchlorate substance as oxidizer are well known and used in this industry.
The invention RU 2215725 describes the explosive composition comprising a perchlorate-type oxidizer, fuel and disruptive explosive, wherein the fuel can be organic non-explosive fuel or metallic fuel.
The invention RU 2190585 teaches about an explosive composition for wells; the composition is a mixture of oxidizer, hexogene, and fuel, wherein ammonium perchlorate is the oxidizer and fuel is aluminum or graphite powder.
However, these technical solutions produce only a single explosion and do not suite for “soft” impact on the wellbore shattering or compaction of the formation. There is no sufficient information about these devices to consider the opportunity to arrange the pulse-type combustion in the wellbore.
There exist several designs of solid-fuel gas generators for spalling of the reservoir. Several patents disclose gas generators based on granulated gun powder and solid propellant: the charges are loaded into a shell. These generators produce only a single fast pressure pulse suitable for creation a multitude of small cracks or one big fracture in the formation, depending on the pressure growth rate (RU2275500, RU2103493, SU912918, RU2175059, SU1574799, U.S. Pat. Nos. 5,295,545, 3,174,545, 3,422,760, 3,090,436, 4,530,396, 4,683,943, 5,005,641). However, the mentioned patents did not disclose the device and the basic composition of the mixture suitable for cyclic pulse mode of propellant combustion.
Patents U.S. Pat. No. 3,422,760 and RU 2204706 disclose the devices operating in pulsed mode due to successive combustion of several separate charges. The patent U.S. Pat. No. 4,530,396 describes the device with two charges having different combustion rates. Patents RU2018508, RU2047744, RU933959, RU2175059 describe different generators without shell: the solid-fuel cylindrical charges are lowered into the well on a cable or slickline and then activated downhole.
Several of mentioned patents describe the situation of pulsing behavior for pressure in the treatment zone after ignition of single charges. This behavior arises due to inertia of wellbore fluid and natural feature of gun powder charges: the combustion rate increases with pressure and decreases as it declines. But none of known designs consider generation of cyclic pressure pulses due to alternating of burning rate for layers of different porosity, where one could produce not a series of consecutive explosions, but rather a process of convective combustion of layers occurring with preselected rates.
The objective of this invention is developing a device and method for formation treatment through generating cyclic pressure pulses with variable amplitude and time characteristics: this series of pulses is localized in space and method ensures convective combustion suitable for “soft” impact upon the wellbore without well damaging and reservoir rock compression.